Rechargeable lithium-ion batteries are attractive energy storage systems for portable electronics and electric and hybrid-electric vehicles because of their high specific energy compared to other electrochemical energy storage devices. A typical Li-ion cell contains a negative electrode, a positive electrode, and a separator region between the negative and positive electrodes. Both electrodes contain active materials that insert or react with lithium reversibly. In some cases the negative electrode may include lithium metal, which can be electrochemically dissolved and deposited reversibly. The separator contains an electrolyte with a lithium cation, and serves as a physical barrier between the electrodes such that none of the electrodes are electronically connected within the cell.
Typically, during charging, there is generation of electrons at the positive electrode and consumption of an equal amount of electrons at the negative electrode, and these electrons are transferred via an external circuit. In the ideal charging of the cell, these electrons are generated at the positive electrode because there is extraction via oxidation of lithium ions from the active material of the positive electrode, and the electrons are consumed at the negative electrode because there is reduction of lithium ions into the active material of the negative electrode. During discharging, the exact opposite reactions occur.
Batteries with a lithium metal negative electrode afford exceptionally high specific energy (in Wh/kg) and energy density (in Wh/L) compared to batteries with conventional carbonaceous negative electrodes. However, the cycle life of such systems is rather limited due to (a) significant volume changes in the cell sandwich during every cycle as the Li metal is stripped and plated, (b) formation of dendrites during recharge that may penetrate the separator and short the cell and/or result in fragmentation and capacity loss of the negative electrode; (c) morphology changes in the metal upon extended cycling that result in a large overall volume change in the cell; and (d) changes in the structure and composition of the passivating layer that forms at the surface of the metal when exposed to certain electrolytes, which may isolate some metal and/or increase the resistance of the cell over time.
When high-specific-capacity negative electrodes such as a metal are used in a battery, the maximum benefit of the capacity increase over conventional systems is realized when a high-capacity positive electrode active material is also used. For example, conventional lithium-intercalating oxides (e.g., LiCoO2, LiNi0.8Co0.15Al0.05O2, Li1.1Ni0.3CO0.3Mn0.3O2) are typically limited to a theoretical capacity of 280 mAh/g (based on the mass of the lithiated oxide) and a practical capacity of 180 to 250 mAh/g, which is quite low compared to the specific capacity of lithium metal, 3863 mAh/g. The highest theoretical capacity for which some practical cycling has been achieved for a lithium-ion positive electrode is 1168 mAh/g (based on the mass of the lithiated material), which is shared by Li2S and Li2O2. Other high-capacity materials include BiF3 (303 mAh/g, lithiated), FeF3 (712 mAh/g, lithiated), LiOH.H2O (639 mAh/g), and others. Unfortunately, all of these materials react with lithium at a lower voltage compared to conventional oxide positive electrodes, hence limiting the theoretical specific energy; however, the theoretical specific energies are still very high (>800 Wh/kg, compared to a maximum of ˜500 Wh/kg for a cell with lithium negative and conventional oxide positive electrodes).
FIG. 1 depicts a chart 2 showing the range achievable for a vehicle using battery packs of different specific energies versus the weight of the battery pack. In the chart 10, the specific energies are for an entire cell, including cell packaging weight, assuming a 50% weight increase for forming a battery pack from a particular set of cells. The U.S. Department of Energy has established a weight limit of 200 kg for a battery pack that is located within a vehicle. Accordingly, only a battery pack with about 600 Wh/kg or more can achieve a range of 300 miles.
Various lithium-based chemistries have been investigated for use in various applications including in vehicles. FIG. 2 depicts a chart 4 which identifies the specific energy and energy density of various lithium-based chemistries. In the chart 4, only the weight of the active materials, current collectors, binders, separator, and other inert material of the battery cells are included. The packaging weight, such as tabs, the cell can, etc., are not included. As is evident from the chart 4, lithium/air batteries, even allowing for packaging weight, are capable of providing a specific energy>600 Wh/kg and thus have the potential to enable driving ranges of electric vehicles of more than 300 miles without recharging, at a similar cost to typical lithium ion batteries.
The appeal of a lithium/air cell is further exhibited by Table 1 below which shows properties of various discharge products of Li, O2, and other species.
Uθ vs.TheoreticalTheoreticalSpecificCapacityLispecific energyenergy densityActiveCapacityDensitydensitymetal(vs. Li metal)(vs. Li metal)Material(mAh/g)(g/cm3)(mAh/cm3)(V)(kWh/kg)(kWh/l)Li2O17942.0136062.915.2210.49Li2O211682.3126982.963.467.99LiOH•H2O6391.519653.452.203.33LiOH11191.4616343.453.865.60LiMO2,2754.2511693.751.034.36M = Mn,Ni, CoLiFePO41703.66123.420.582.09Li metal38610.53420620.0
While lithium/air cells have been demonstrated in controlled laboratory environments, a number of issues remain before full commercial introduction of a lithium/air cell is viable. Thus, while lithium-based batteries have a sufficiently high specific energy (Wh/kg) and energy density (Wh/L) that they are now being used in electric-powered vehicles, in order to power a full-electric vehicle with a range of several hundred miles, a battery with a higher specific energy than the present state of the art (an intercalation system with a graphite anode and transition-metal oxide cathode) is necessary. For example, the lithium-oxygen battery, which uses a lithium metal negative electrode and a positive electrode that reacts O2 and H2O to form LiOH (which has a solubility limit in H2O of ˜5.2 M at room temperature) and LiOH.H2O (which precipitates out of solution once the solubility limit has been exceeded), has a significantly higher specific energy than the present state of the art.
Reactions among Li and O2 to give high-energy products may be carried out in a number of chemical media and with additional reactants, as shown in Table 2 below:
Reactants/ReactionCommentsNon-aqueous2Li + ½O2   Li2ONot typically observed2Li + O2    Li2O2Observed, no breaking the O—Obond, Li2O2 is reactive withcarbon and many solventsAqueous: alkaline2Li + ½O2 + H2O    2LiOHLiOH saturates at ~5.2M, limitsenergy storage2Li + ½O2 + 3H2O    2LiOH•H2OPrincipal reactant isH2O! 6 H2O/O2, 1.5 H2O/LiAqueous: “neutral”2Li + ½O2 + 3H2O    2LiOH•H2OCl2 evolution possible for(in concentrated LiCl or LiNO3)LiCl, extra salt adds mass andmay also precipitateAqueous: acidic2Li + ½O2 + 2AcOH  Lower theoretical energy2AcOLi + H2Othan Li2O2, LiOH•H2O
As two specific examples, in a non-aqueous medium the products Li2O2 and Li2O may form, while in a basic aqueous medium LiOH (dissolved) and eventually LiOH.H2O (precipitated) may form. In all of the cases shown in Table 2, the Li metal is consumed during discharge, leading to a significant volume change. In particular, for Li metal about 4.85 microns of Li metal are reacted per mAh/cm2 of capacity, and because the target capacity is 20 mAh/cm2 and above, a thickness change of the Li metal of at least 100 microns is required, although 200 microns or more (including some Li metal in excess) is desirable.
The discharge products shown in Table 2 have different densities and specific capacities, and may increase the volume of the region in which they are grown during discharge, depending on the composition of that region (e.g., whether it includes a gas phase that can be displaced and the mechanical boundary conditions of that region (e.g., whether displacement is prevented by a mechanical blocking layer).
Significant volume changes during cycling induce mechanical stresses within the individual regions in which the volume change takes place, as well as the overall cell. Other challenges include reducing the hysteresis between the charge and discharge voltages (which limits the round-trip energy efficiency), improving the number of cycles over which the system can be cycled reversibly, limiting dendrite formation at the lithium metal surface, protecting the lithium metal (and possibly other materials) from moisture and other potentially harmful components of air, and designing a system that actually achieves a high specific energy and has an acceptable specific power.
What is needed is a cell wherein the amount of volume change in the cell sandwich is reduced.